Digital materials are comprised of a small number of types of discrete physical building blocks, which assemble to form constructions that meet the versatility and scalability of digital computation and communication systems. Digital materials promise scalable methods of producing functional things with reconfigurable sets of discrete and compatible parts.
The science of cellular solids has enabled the widespread use of lightweight materials to meet important engineering needs, such as passive energy absorption, but they are not in widespread use for structural applications, perhaps due to a large gap between the strength and stiffness to weight ratios of popular classical solids, and the performance of known lightweight cellular materials that are produced from the same constituent material. The engineering of fiber reinforced composite materials has enabled structures with large reductions in weight for given strength and stiffness targets, but at very high design and processing costs, and many challenges in producing mechanical interfaces (joints). Continuously shape-morphing structures have mostly focused on traditional kinematics with flexural components that match or exceed the deformation length scales, and/or rely on high density and high cost materials such as piezoelectric ceramics, shape memory alloys, and electro-active polymers. This has limited the size, degrees of freedom, and manufacturability of shape-morphing structures to date.
Conventionally designed and engineered fabrication methods employ digital computation and communication algorithms to control analog mechanical equipment that additively or subtractively forms shapes from masses of bulk material. Digital material systems instead propose a method for fabrication from discrete parts with discrete relative local positioning, instead of continuous variation of composition and location of material, as in an analog fabrication system. This may be thought of as printing, noting that an important distinction between digital material printing and conventional commercially available three dimensional printing processes is that digital material printing is reversible, and the information regarding the shape, assembly, and function of a finished product is intrinsic to the material that it is composed of.
Structure design and construction requires consideration of multiple factors. The design and fabrication process will generally include considerations of: 1) design requirements, 2) likely failure modes, 3) stress analysis for failure modes identified, 4) material selection and behavior, 5) fabrication, and 6) testing, all within the context of the overall design goals. For example, in order to achieve reduction in weight, increase in strength, and reduction in cost, the engineering design, materials of construction, and methods of fabrication must all be considered. In general, modern fabrication techniques include various additive and subtractive processes, employing a range of materials, including, but not limited to, composite materials, cellular materials, and digital materials.
“Composite materials” describes any two materials which are combined together in a single bulk material to obtain the best properties from both materials. Many industries are shifting towards the use of more composite materials because they display the single most significant consideration for any application: low weight compared to strength. The material properties of composites are unlike any material thus far, because they combine the properties of a high modulus and high tensile strength fiber for flexibility and strength, with a low modulus stiff matrix which transfers forces from one fiber to the next, creating essentially a continuous analog bulk material. Fiber-reinforced composite materials have thus enabled construction of structures having large reductions in weight for given strength and stiffness targets, but this reduction comes at the cost of very high design and processing costs and many challenges in producing mechanical interfaces (joints).
Composites are still problematic as the material of choice hindering widespread use for many reasons. First, composites vary in fibers, resins, and weaves from one manufacturer to the next, with strength and weight dependent on layup and direction of weave. Second, composites require an energy intensive process. Highly skilled technicians are never really able to have complete control over the application of pressure and heat to allow for proper curing and even distribution of heat over the entire surface [Dorworth, C. Louis, Gardiner L. Ginger, Mellema M. Greg, “Essentials of Advanced Composite Fabrication and Repair”, 2010]. Third, any flaw detected in a composite skin renders the entire material a complete waste, or makes repair difficult since creating the exact conditions to maintain bond strength is close to impossible to achieve. Fourth, not only is the composite surface designed, but the tooling and moulding for the composite is just as intensive as the final part. In the process of mitigating stress concentration, composite skins are ultimately labor intensive, time intensive, and expensive.
“Cellular materials” or “cellular solids” refers to the material structure of any living or nonliving matter, typically described as anisotropic and unidirectional or isotropic and having the same properties in all directions. Cellular materials can fill space in two-dimensions as extruded honeycomb or prismatic cells or three-dimensions as space filling polyhedra in various lattice formations. Cellular materials have been mimicked in engineered foam core structures used in construction, aerospace, and medical industries. These man made materials can be designed as highly porous scaffolds or fully dense structures which can be mechanically tuneable for a specific performance. While the science of cellular solids has enabled widespread use of lightweight materials to meet many important engineering needs, such as passive energy absorption, cellular solids are not presently in widespread use for structural applications, perhaps due to a large gap between the strength and stiffness to weight ratios of popular classical solids and the performance of known lightweight cellular materials produced from the same constituent material.
Material science has made much progress in the description of natural cellular solids, such as wood and bone [Gibson, L. J., “Biomechanics of cellular solids”, Journal of Biomechanics, vol. 38, pp. 377-399, 2005]. Such biological cellular solids may be considered to be a prime example of natural structural fabrication that is high performing with complex constraints. For analysis, it is now typical to treat cellular materials as classical solids. The properties of the cellular solid are therefore defined by the properties of the solid material that it is made from (the “constituent solid”), and its spatial configuration [Gibson, L., & Ashby, M., Cellular Solids, Structure and Properties, Cambridge University Press., Cambridge, UK, 1988].
Much of the art of continuum mechanics of cellular solids lies in developing a classically analyzed cell model that is an effective representation of the stochastically varying nature of the actual material. The field has done quite well to characterize readily available natural cellular solids in this manner. Natural scaling laws are well known [Gibson, L., & Ashby, M., Cellular Solids, Structure and Properties, Cambridge University Press., Cambridge, UK, 1988], and relate the mechanical properties of the cellular solid to those of the constituent solid material (that which comprises cell edges and/or walls), via the relative density of the former to the latter.
A large variety of applications have developed around the science of cellular solids, evidenced by the widespread use of these materials for passive energy absorption, thermal insulation, and fluid filtering [Maiti, S. K., Gibson, L. J., & Ashby, M. F., “Deformation and Energy Absorption Diagrams for Cellular Solids”, Acta Metall, Vol. 32, no. 11, pp. 1963-1975, 1984]. However, despite their low density, they are largely limited in their use for structural applications, because for the weight of popular and inexpensive cellular solids, they are not particularly strong. The conventional model typically considers the geometry of stochastic foams to be such that transverse beam bending dominates the behavior of the material [Gibson, L., & Ashby, M., Cellular Solids, Structure and Properties, Cambridge University Press., Cambridge, UK, 1988]. This results in an overall relative modulus that is expected to be proportional to the square of the relative density, for natural foams.
                                          E            *                                E            s                          ∝                              ρ            2                                ρ            s            2                                              Equation        ⁢                                  ⁢        1            
In additional to biological cellular solids, this scaling law is useful for analyzing and predicting the properties of non-biological natural foams (e.g. pumice) and engineered foams (e.g. polymer foams, metal foams), whether chemically or mechanically produced. Further analogies to natural cellular structures exist in conventional engineered structures [Aizenberg, A., Weaver, J., Thanawala, M. S., Sundar, V. C., Morse, D. E., & Fratzl, P., “Skeleton of Euplectella sp.: Structural Hierarchy from the Nanoscale to the Macroscale”, Science, vol. 309, pp. 275-278, 2005]. Depending on the relative scale desired for analysis, one might look towards the aforementioned engineered foams, or space frame trusses and aero-structures.
Architecture and civil engineering have employed space frame truss structures for many years. These have not previously been scaled volumetrically, as a perfect lattice, to the orders of units that make it practical to consider the bulk assemblies as a continuum, as would be beneficial for engineering and design purposes. Further, it is well known that space frames with many elements sharing structural duty possess highly desirable characteristics in terms of failure modes and damage tolerance [Lakes, R., “Materials with structural hierarchy”, Nature, vol. 361, pp. 511-515, 1993; Huybrechts, S., & Tsai, S. W., “Analysis and Behavior of Grid Structures”, Composites Science and Technology, vol. 56, pp. 1001-1015, 1996]. This is evident in “geodetic” airframe designs [Paul, D., Kelly, L., Venkaya, V., & Hess, T., “Evolution of U.S. Military Aircraft Structures Technology”, Journal of Aircraft, vol. 39, no. 1, pp. 18-29, 2002]. The current state of robotic manufacturing technology makes it easy to see how massively parallel assembly of digital materials can be implemented, including the assembly of structures that are larger than the assembly machinery.
The commercial aerospace industry has been moving towards aircraft designs that have fewer but larger monolithic fiber composite parts, in order to produce highly tuned and lightweight structural systems that meet extreme service, monitoring, and general environmental requirements. Conventional wisdom is that larger monolithic parts are better than an assembly of smaller parts because producing effective joints between parts is highly problematic in practice. Conventional manufacturing processes have scaled up, accordingly, which requires tools (e.g., molds for defining the shape of the part), and ovens (e.g., autoclaves for polymer matrix curing) that are large enough to influence the size of the buildings that must contain them. Some may consider that the expense involved with these manufacturing methods limits the industry altogether; there is no question that it limits prototyping capabilities. Further, the per-part investment is high enough to warrant complex repair processes as defects of small relative size arise, to say nothing of their contribution to resource intensive qualification procedures [U.S. Department of Defense, Composite Materials Handbook, “Polymer Matrix Composites Guidelines for Characterization of Structural Materials”, MIL-HDBK-17-1F 1, 2002].
These relatively recent methods also rely on a basis set of conventional manufacturing and fabrication tools and processes. Conventional subtractive manufacturing processes take solid blocks or sheets of material and machine out material by drilling or milling from the existing material to create the final part. The initial material is analog in nature, but often these discrete parts are combined within larger assemblies using irreversible joining and bonding methods which again, render the assemblies irreversible, with surface resolution depending on the machine tools used, and any error in the part means waste of the entire assembly of materials. Conventional subtractive techniques (i.e. milling, water-jet cutting) work poorly with fiber reinforced polymer materials (i.e. requiring diamond tipped bits for carbon fiber composites, subject to wetting and de-lamination during abrasive water jet cutting). For any given additive or subtractive process, representation of the initial model and translation from initial design to final product requires greater integration than the tools currently offer
Several additive methods that use discrete components to create an analog material as a final product are known in the art. Conventional additive fiber reinforced polymer manufacturing techniques involve dynamic weaving and robotic layup about formwork that is the size of a part (or larger), requiring very large investments in tooling. Other conventional rapid prototyping technologies such as additive computer controlled three dimensional material printing processes do not produce structurally tuned fiber reinforced composite parts. Selective laser sintering (SLS) uses high power lasers to fuse powders such as glass, metal or thermoplastics, creating forms that are irreversible. The powders are not analog, but are initially formless particles that are discrete and separate. Upon fusing a particle to another, a new analog material is created that is continuous and attached to adjacent particles to form the larger object. Another such additive method is fused deposition modeling (FDM). FDM takes a coil of thermoplastic or metal wire and deposits material from an extruder by heating and melting the material. Stereolithography (SLA) is similar to SLS, but instead of using powder, uses a vat of liquid with a high power laser to create the part in cured layers [Bourell, D. L., Leu, M. C. & Rosen, D. W (Eds.), Roadmap for Additive Manufacturing: Identifying the Future of Freeform Processing, Austin, Tex., The University of Texas at Austin Laboratory for Free-form Fabrication, 2009]. Electron beam melting (EBM) is another additive process prevalent in the aerospace industry, and uses an electron beam to melt metals such as titanium in powder form. Similar to previous processes, each part is built one layer at a time, solidified and a subsequent layer is built. Current additive manufacturing technologies may utilize the same materials used in manufacturing processes, but the final products rarely behave per material specification, always depend on the machine for surface resolution, and any error in the part generates wasted material.
The aforementioned advances of material science in engineering of cellular solids, such as honeycomb core materials and foams, have resulted in the ability to design with lighter, more elastic, more insulating, and more energy absorptive materials. The practice of treating cellular solids as conventional continuous solids allows for simple application with conventional engineering and design methods. In the context of cellular materials, it has been noted that “constructed” periodic metal lattices allow for much larger cell size, and therefore lower relative density, compared to other methods of producing cellular metals [Wadley, H., “Cellular Metals Manufacturing”, Advanced Engineering Materials, vol. 4, no. 10, pp. 726-733, 2002].
A natural result—of the understanding and application of cellular material property scaling laws—has been an interest in ultra-light materials. Ultra-light materials may be considered to include any material that is less than 0.1 grams per cubic centimeter. These materials are generally known to obey a less desirable scaling than the denser stochastic cellular materials [Schaedler, T. A., Jacobsen, A. J., Torrents, A., Sorensen, A. E., Lian, J., Greer, J. R., Valdevit, L., & Carter, W. B., “Ultralight Metallic Microlattices”, Science, vol. 334, pp. 962-965, 2011], as in Equation 1.
                                          E            *                                E            s                          ∝                              ρ            3                                ρ            s            3                                              Equation        ⁢                                  ⁢        2            
While this generally applies to aerogels, recent results have shown processes that achieve the quadratic scaling of denser stochastic cellular materials (Schaedler, T. A., Jacobsen, A. J., Torrents, A., Sorensen, A. E., Lian, J., Greer, J. R., Valdevit, L., & Carter, W. B., “Ultralight Metallic Microlattices”, Science, vol. 334, pp. 962-965, 2011; Mecklenburg, M., Schuchardt, A., Mishra, Y. K., Kaps, S., Adelung, R., Lotnyk, A., Kienle, L., & Schulte, K., “Aerographite: Ultra Lightweight, Flexible Nanowall, Carbon Microtube Material with Outstanding Mechanical Performance”, Advanced Materials, vol. 24, pp. 3486-3490; 2012].
Digital materials promise scalable methods of producing functional things with reconfigurable sets of discrete and compatible parts. Digital materials are comprised of a small number of types of discrete physical building blocks that may be assembled to form constructions that have a level of versatility and scalability that is analogous to that of digital computation and communication systems. Digital materials have specifically been defined in prior work by Popescu as having three main properties at the highest level of description: a finite set of components or discrete parts, a finite set of discretized joints of all components in a digital material, and complete control of assembly and placement of discrete interlocking components [Popescu, G., Gershenfeld, N. and Marhale, T., “Digital Materials For Digital Printing”, International Conference on Digital Fabrication Technologies, Denver, Colo., September 2006].
A digital material desktop printer, now called the MTM Snap, was the first application constructed entirely out of discrete, snap-fit, reversible digital materials The entire structure for the MTM Snap is made up of a finite set of discrete parts, with built-in flexural connections and slots that are all milled as one CAD file on any CNC shopbot machine. The parts for the machine are made of high density polyethylene, which as a material demonstrates great potential to create robust and stiff flexural connections, although it can be made out of many other suitable materials. The entire machine can be fabricated within a day, with additional motors and tool heads installed depending on the fabrication method desired. These digital material printers can print or mill their own parts, in order to replicate and build more machines like themselves. Current work at MIT's Center for Bits and Atoms is taking the digital material printer to the next level, by incorporating a pick and place mechanism, called a digital material assembler, which is a machine that picks and places each newly fabricated piece to create the final form.
Digital Cellular Solids are cellular solids that exhibit improvements in relative stiffness and strength compared to relative density, over current practices for producing lightweight materials. This is accomplished by assembling lattice geometries that perform better than any currently made with traditional methods. When implemented with fiber composites, the result is not only stiffer and stronger than any previously known ultra-light material, but it presents a new scalable and flexible workflow for applying fiber composites to engineering problems.
Digital composites would allow for rapid prototyping of fiber composite parts with high throughput robotic digital assemblers. The individual components may be produced through conventional means, as suited for mass production of identical parts. With digital assembly of sparse volumes composed of many smaller components, all of the tooling required may be significantly smaller than the finished assemblies. The possible properties of digital materials are myriad, and they can be designed out of any material using existing fabrication technologies and tools in order to build cellular structures for any application. Digital materials, as compared to analog materials, are completely reversible, eliminating waste by allowing individual parts to be reused and recycled at any point in the product lifecycle, no matter how large the assembly.